1. Field of the Invention
This invention relates to an improved friction element containing a binder comprising the reaction product of phenol, formaldehyde, and monoalkylphenol and a method of making such a friction element employing such a binder.
2. Prior Art
Various mechanical devices which are movable or have movable parts utilize frictional forces to transmit, slow or stop motion. This is accomplished by contacting a moving part with an immobilized part until the frictional forces between the contacting parts cause the moving part to slow sufficiently or to stop. Such devices are customarily referred to as brakes. A clutch on the other hand utilizes such frictional forces for engaging and disengaging movable parts.
Brakes for vehicles are of two main types, "disc" brakes and "drum" brakes. Disc brakes employ a disc or a plate located on the inboard face of a wheel and have positioned on opposite sides of the disc non-rotating brake shoes fitted on their contacting surfaces with friction pads of brake lining material. Mechanical or hydraulic linkages are provided to force the friction pads into engagement with the rotating disc and to release such engagement.
Drum brakes utilize non-rotating brake shoes having generally curved friction pads or brake linings on their surfaces which are forced into engagement with a brake drum which rotates with the wheel. The shoes are mechanically or hydraulically forced against the drum when appropriate braking force is applied. In each case, the normal position of the friction pad is that of being disengaged from the drum or disc.
The surface contacted by the brake lining is typically a hard, smooth, essentially non-wearing part, customarily made of steel, cast iron or aluminum. The friction pad is somewhat softer than the surface which it contacts, more resilient and adapted to provide the appropriate frictional surface. Such pads are typically replaceable and generally slowly wear away so they must be periodically replaced.
The necessary performance characteristics of the brake lining material forming such friction elements include dimensional stability, a relatively constant friction level as temperature is varied since heat is always generated in the braking operation, low lining wear and minimum wear of the brake drum or disc against which it is applied.
Such friction elements are generally formed of a fibrous thermally stable reinforcing material, various metallic or non-metallic powdered or particulate solid filler materials and a binder resin to hold the mass together. For the purpose of this application the ingredients contained in the friction elements other than the binder shall be referred to as "filler". This term is intended to include such conventional ingredients as particulate or powdered friction material; powdered, particulate or fibrous reinforcing materials; additives to assist in the dissipation of heat; and the like. The binder typically comprises about 5% to about 25% of the friction element with the balance of 95% to 75% being such filler.
There are two popular processes currently employed for the manufacture of such friction elements. The first involves mixing a powdered solid reactive cross-linkable B-stage binder resin with the requisite filler and optionally a suitable curative in a mixing device such as a blender. (Such curatives are compounds or mixtures which provide a reactive species to cross-link or cause cross-linking of the resin. Commonly used curatives include sulfur and formaldehyde donor compounds, such as hexamethylene tetramine.) The mix, which is loose and pourable, is first pre-formed and then transferred to a mold having a cavity of the appropriate size and shape wherein the mixture is heated and pressed until the binder resin flows to coat the filler and then cures producing a mass which may be machined, if required, to the appropriate shape.
The second process utilizes a liquid resin binder, dissolved in a suitable solvent if required, which is mixed with the desired filler and curative if needed, in a mixer to provide a homogeneous doughy mass. The mass is then processed into the appropriate shape by extrusion or roll molding to produce an uncured, or "green", shape which is dried to remove solvent, if used, then heated in an unconfined state to cure the binder.
Brake linings made with presently available liquid binders suffer some shortcomings. The major problem is a loss of effectiveness upon continuous heavy use of the brake, called "fade". This problem is apparent to the operator of a motor vehicle as an increase in the amount of pedal effort needed to slow or stop the vehicle. Fade is typically traceable to a decrease in the coefficient of friction as the brake linings absorb the heat of repeated stops, forcing the lining temperature higher and higher. This shortcoming leads to unpredictable and unsafe operating conditions. Laboratory testing of the brake linings made from such conventional resins confirm that they exhibit low friction at high temperature. In some cases, the coefficient of friction drops below 0.2 at temperatures as low as 650.degree. F.
There are three main types of liquid binder resins presently utilized in the second process to produce the majority of brake linings. The first type is based on linseed oil which has been "bodied", i.e., modified to increase its molecular weight and viscosity. Such modification is typically carried out by heating the linseed oil in the presence of oxygen and/or a catalyst, causing some of the oil molecules to link together to form a cross-linkable reactive resin containing unsaturation which is curable in the presence of a sulfur curative. The second type liquid resin is based upon cashew nut shell liquid which has been "bodied" by heating in the presence of a strong acid. This results in a resin which contains both carbon- carbon unsaturation curable with sulfur or sulfur-containing compounds and reactive phenolic groups curable with formaldehyde donor compounds such as hexamethylene tetramine (commonly known as "Hexa"). The third type of binder liquid is an oil-modified phenolic resin which is prepared from condensation of phenol with various vegetable oil materials which soften and solubilize otherwise hard and insoluble phenolic resins. Some of the binders disclosed herein may be known in the chemical art but, as far as is known to applicant, none of the binders defined herein are known for use in friction elements. For example, Japanese Pat. No. 47-13621 discloses a resin prepared by the base catalyzed reaction of nonylphenol and formaldehyde with phenol, useful as a molding resin for electrical insulators and the like.